1. Field of the Invention
The present invention relates to an inverter apparatus for driving a cold cathode fluorescent lamp (CCFL), and particularly, to a technique of suppressing the brightness variation and flicker of a CCFL.
2. Description of the Related Art
FIG. 1 shows a cold cathode fluorescent lamp inverter apparatus (hereinafter referred to as CCFL inverter apparatus) according to a related art. In the CCFL inverter apparatus, an AC power source AC provides an AC voltage. The AC voltage is rectified by a diode bridge circuit DB and is smoothed by a capacitor Cin into DC power serving as a DC power source. The DC power is supplied to a series circuit consisting of a first switching element Q1 made of a MOSFET and a second switching element Q2 made of a MOSFET. The first and second switching elements Q1 and Q2 are turned on/off according to control signals provided by a controller 10.
Between the drain and source of the second switching element Q2, there are connected a voltage quasi-resonant capacitor C6 and a series circuit including a reactor Lr1 and a current resonant capacitor C4. The reactor Lr1 is a leakage inductance between a primary winding P1 of a transformer T1 and a secondary winding S1 thereof.
In FIG. 1, the leakage inductance between the primary and secondary windings P1 and S1 of the transformer T1 serving as the reactor Lr1 is a reactor Lr1a (depicted with a dotted line) on the primary side of the transformer or a reactor Lr1b (depicted with a dotted line) on the secondary side thereof. The inductance controls a resonant operation. The inductors Lr1a and Lr1b may be separate inductors, or may be connected to the primary side of the transformer, or may be connected to the secondary side thereof, or may be connected to both sides thereof. Ends of the secondary winding S1 of the transformer T1 are connected in series with a cold cathode fluorescent lamp (CCFL) 20 and a current detector 30.
Operation of the CCFL apparatus having the above-mentioned configuration will be explained. In response to control signals from the controller 10, the first and second switching elements Q1 and Q2 conduct a switching operation. The first and second switching elements Q1 and Q2 are controlled so that they are turned on/off alternately with each other and so that they simultaneously have an OFF period. The ON/OFF control of the first and second switching elements Q1 and Q2 may be PWM (pulse width modulation) control, phase control, or frequency control.
The ON/OFF operation of the first and second switching elements Q1 and Q2 intermittently cuts a DC voltage supplied from the DC power source to the respective switching elements, to thereby apply an AC voltage to the primary winding P1 of the transformer T1. As a result, the secondary winding S1 of the transformer T1 generates an AC voltage to pass an AC current through the CCFL 20 and current detector 30.
The current detector 30 detects the current passing through the CCFL 20 and sends a feedback signal Sfb to the controller 10 on the primary side. The current detector 30 has an I/O terminal Iac1 connected to the CCFL 20, an I/O terminal Iac2 (GND) connected to the secondary winding S1 of the transformer T1, and an output terminal Vco connected to the controller 10. In response to the feedback signal Sfb from the output terminal Vco of the current detector 30, the controller 10 controls the ON/OFF operation of the first and second switching elements Q1 and Q2, to thereby control an AC voltage applied to the primary winding P1 of the transformer T1 so that a current having a predetermined value passes through the CCFL 20.
FIGS. 2A to 2C show examples of the current detector 30 according to related arts. In FIG. 2A, the current detector 30 has diodes D51 and D52 that are oppositely connected between the I/O terminals Iac1 and Iac2. The diodes D51 and D52 pass AC currents of opposite polarities, respectively. The diode D51 is connected in series with a resistor R51. The resistor R51 is connected, through a diode D50, in parallel with a capacitor C51 and a resistor R50. A first end of the capacitor C51, i.e., a connection point to the diode D50 serves as the output terminal Vco. A current passing through the CCFL 20 via diodes D51 and D50 flows to the capacitor C51, so that a peak value of the current to the CCFL 20 is accumulated in the capacitor C51. The resistor R50 works as a discharge resistor of the capacitor C51. The current detector 30 in FIG. 2A is a peak-current detector.
In FIG. 2B, the current detector 30 has diodes D51 and D52 that are oppositely connected between the I/O terminals Iac1 and Iac2. The diodes D51 and D52 pass AC currents of opposite polarities, respectively. The diode D51 is connected in series with a series circuit including resistors R52 and R51. The resistor R51 is connected in parallel with a capacitor C51. A first end of the capacitor C51, i.e., a connection point of the resistors R52 and R51 serves as the output terminal Vco. A current passing through the CCFL 20 via the diode D51 and resistor R52 flows to the capacitor C51, to charge the capacitor C51. Namely, the capacitor C51 is charged and discharged with an average of the current to the CCFL 20, i.e., a voltage divided by the resistors R51 and R52. The current detector 30 of FIG. 2B is an average-current detector.
In FIG. 2C, the current detector 30 has diodes D51 and D52 that are oppositely connected between the I/O terminals Iac1 and Iac2. The diodes D51 and D52 pass AC currents of opposite polarities, respectively. The diode D51 is connected in series with a resistor R51. A connection point of the diode D51 and resistor R51 is connected through a resistor R52 to an inverting input terminal of an operational amplifier OP51. A non-inverting input terminal of the operational amplifier OP51 receives a reference voltage Vr51. Connected between the inverting input terminal of the operational amplifier OP51 and an output terminal thereof is a capacitor C51, to form an integration circuit. The output terminal of the operational amplifier OP51 serves as the output terminal Vco of the current detector 30. The current detector 30 of FIG. 2C is an average-current detector using an operational amplifier.
Another related art is disclosed in Japanese Unexamined Patent Application Publication No. H11-26178. This disclosure is a charge-pump-type discharge lamp lighting apparatus. The apparatus includes an inverter circuit having two switching elements to convert a terminal voltage of a smoothing capacitor into high-frequency power. An output from the inverter circuit is supplied through a resonant circuit to a discharge lamp. Between a rectify circuit and the resonant circuit, a capacitor is connected. A feedback circuit feedback-controls an ON/OFF operation of the switching elements by modulating control signals provided by a controller within an allowable range in such a way as to reduce a ripple in a lamp current detected by a current detector. A synthesizer corrects the lamp current to the feedback circuit according to a dimming signal, to prevent an increase in the ripple of the lamp current due to dimming.